This invention relates to a shroud for gas turbines which is mounted on the inside of a turbine casing so as to face the tips of moving blades with a gap therebetween, and more particularly to the shroud made of austenitic, heat-resistant, Fe-Ni-Cr-base alloy steel which is superior in thermal fatigue resistance and creep rupture strength.
The shroud for gas turbines, which is exposed to a corrosive gas at one side and to coolant at the other side, easily suffers repeated thermal stress which leads to deformation or cracking. Further, the shroud faces the tips of the moving blades with a small gap, so that its deformation causes the danger that the shroud contacts with the moving blades. When a shroud for gas turbines is repeatedly subjected to a corrosive gas of high temperature, large thermal stress is repeatedly produced thereby.
Recently, highly efficient gas turbines which use higher temperature gas than that used in a conventional gas turbine have been developed, and metal temperature of the shroud has reached to a temperature of 650.degree.-900.degree. C. For the shroud, Fe-25Cr-20Ni alloy steel equivalent to CK20 has been used. Generally, the shroud is subjected to high thermal stresses which lead to deformation or cracking. It was found that the CK20 alloy is prone to crack from thermal fatigue and its life is shortened.
On the other hand, blades for gas turbines are exposed to atmosphere of higher temperature than the shroud. Alloy of higher strength for high temperature use, for example, nickel-base alloy or cobalt-base alloy disclosed in U.S. Pat. No. 4,169,020, is used. These materials, however, include a lot of expensive Ni and Co and their thermal fatigue property is superior to that of the CK20. However, the shroud made of Ni-base or Co-base alloy is very expensive.